1. Field of the Invention
The present invention relates to an X-ray exposure apparatus which exposes a semiconductor wafer or the like using X-rays that are generated by the radiation of a synchrotron (SOR) or electron-ray excitation, or X-rays that are generated by plasma or the like.
2. Description of the Related Art
Recent semiconductor devices are designed with higher integration so that the minimum line width of the pattern of a VLSI (Very Large Scale Integration) circuit reaches the order of submicrons. Exposure apparatuses, which are used in fabricating VLSI devices, expose a semiconductor wafer or the like using X-rays that are generated by the radiation of a synchrotron (SOR) or electron-ray excitation, or X-rays that are generated by plasma or the like.
Such an X-ray exposure apparatus is designed in such a way that X-rays, which are generated from an X-ray source placed in vacuum and are led into a chamber filled with gaseous helium through a window formed by a beryllium foil, transfers a mask pattern on a wafer, placed in the air, through an X-ray mask.
Since the attenuation of X-rays is significant in this X-ray exposure apparatus, it is necessary to keep the low-attenuation atmosphere for X-rays (helium atmosphere) at high purity. As the X-ray mask are very thin, the difference between the pressure in the chamber and the atmospheric pressure should be controlled at high precision in order to prevent those members from being deformed or damaged.
A helium chamber for an X-ray exposure apparatus, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 1-181518 and Jpn. Pat. Appln. KOKAI Publication No. 1-181521, has been proposed as one conventional technique of controlling the chamber pressure.
In this conventional technique, the difference between the chamber pressure and atmospheric pressure can be controlled to about +3 mmH.sub.2 O or +0.2 mmHg due to the difference in specific weight between H.sub.2 O and Hg being 1:13.6). However, the present inventors have conducted various experiments and simulation and found that with the difference between the chamber pressure and atmospheric pressure being 0.2 mmHg, a typical X-ray mask, e.g., an X-ray mask having a membrane thickness of 1 .mu.m and a size of about 25 mm on each side, causes deformation of about 15 .mu.m. In the X-ray exposure apparatus, it is considered to perform a close exposure with the gap between the mask and the wafer being set to about 30 .mu.m. If the mask deforms 15 .mu.m for the 30-.mu.m gap, accurate exposure will not be accomplished.
In a pressure control of about 0.2 mmHg, as described above, since the amount of mask deformation is too large, an accurate exposure cannot be realized. For this reason, finer control on the pressure difference is demanded.
Also, in a case of monitoring oxygen concentration, if the oxygen concentration in the chamber is measured by an oxygen monitor with a suction pump for sucking sample gas, the pressure in the chamber varies. For this reason, it is difficult to satisfy the above demand for the pressure control.